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Generate a list of five popular apps to use for online education.


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1. Udemy: a platform offering thousands of online courses taught by experts;
2. Khan Academy: a non-profit organization offering free educational resources in various subjects;
3. Coursera: a platform with hundreds of top-tier universities and organizations offering online courses and specializations;
4. edX: an open-source learning platform created by Harvard and MIT;
5. Duolingo: an popular app offering interactive language courses.